	#include<iostream>
	#include<cstdio>
	#include<cmath>
	using namespace std;
	int main()
	{
		int n, m;
		double p, q, rate = 0;
		while (scanf("%d%d%", &n, &m) != EOF) {
			scanf("%lf%lf", &p, &q);
			if (n == 0) {
				//cout << 0.00 << endl;
				printf("0.00\n");
				continue;
			}
			if (m == 0) {
				//cout << 1.00 << endl;
				printf("1.00\n");
				continue;
			}
			if (p == 0 || q == 1) {
				//cout << 0.00 << endl;
				printf("0.00\n");
				continue;
			}
			if (q == 0 || p == 1) {
				//cout << 1.00 << endl;
				printf("1.00\n");
				continue;
			}
			//double rate = 0.0;
			if (p == q) {
				rate = 1.0 * n / (n + m);
			}
			else {
				double k = q * (1 - p) / (p * (1 - q));
				rate = (1 - pow(k, n)) / (1 - pow(k, m + n));
			}
			printf("%.2lf\n", rate);
		}
		return 0;
	}